The present invention relates to a process and relevant apparatus for measuring the coaction tensile stress in riveted joints, with particular reference to the riveted joints of cages of straight roller bearings of large dimensions used in railway systems.
In the straight roller bearings of large dimensions, a widely used configuration of the roller guiding cage is that in which the junction is obtained by riveting a ring of rectangular cross-section, referred to as washer, to another ring, referred to as "tenon ring", rigidly connected to as many tenons as are the rollers between which they are interposed. Thus a thick cylindrical zone is created which has openings in the which the rollers are free to rotate.
In the past, the sturdiness and hence the life of the cage was out affected by the stresses which are generated by modern rotation speeds, and the bearing reached the end of its life as result of the wear of the steel which its parts are made of. In fact, the materials and the working were not at such high quality and design levels as they are at present time. In the last several years, the straight roller bearings, better constructed, but subjected to higher stresses, are failing because of loosenings and subsequent breaking of the rivets which, extend into the interior of the tenons to ensure the junction of the washer with the tenon ring. Since it is a fatigue failure, which brings on a rapid seizure of the bearing with consequent breaking of the mechanical members on which it is keyed, it is obviously very important for the rivets to have a mechanical pre-tension sufficient to always ensure a rigid frictional coupling between the said two parts forming the cage. Therefore it is necessary to verify that thereas is mechanical pre-tension of the rivets by rapid and direct measuring during both the manufacturing and the maintenance operations on the bearings. Today, this verification is impossible for the bearings being used, and in many manufacturing firms the control is carried out at very long intervals, by indirect methods which destroy the piece under examination; thus, the desired measuring is carried out by a destructive, and unreliably precise process.